In general, lithographic printing materials are imagewise exposed to harden exposed areas and after allowing unexposed areas to be eluted, the materials are further subjected to washing and finisher gum treatments to obtain a lithographic printing plate. Recently, there have been studied methods of preparing a lithographic printing plate, in which laser digital exposure is conducted to achieve high resolving power and enhanced sharpness, followed by being developed to obtain a lithographic printing plate. For instance, there is known a system, in which, using exposure light sources modulated by image signals transmitted through communication networks or signals outputted from electronic plate making systems or image processing systems, photographic materials are directly subjected to scanning exposure to prepare a lithographic printing plate.
However, conventional lithographic printing materials involved problems that it was difficult to achieve spectral sensitization or speed enhancement in combination with the oscillating wavelength of laser light used for digital exposure.
Recently, lithographic materials used for planographic printing plates, having a photo-polymerizable light-sensitive layer containing a photopolymerization initiator, which can achieve enhanced speed suitable for laser light have been noted for use in digital exposure using laser light. Enhancement of speed to shorten the recording time is desired for CTP (Computer To Plate) printing material in which digital data are recorded using a laser light source. Further, printing plates with improved press life have been desired in various printing fields including newspaper printing and commercial printing such as an advertising medium.
There have been studied means for employing photopolymerization to achieve enhancement of speed. For instance, there was proposed the use of trichloromethyl group containing s-triazine compounds as a photopolymerization initiator described in JP-A Nos. 48-36281, 54-74887, 64-35548 (hereinafter, the term, JP-A refers to Japanese Patent Application Publication); the use of iron arene complex compounds described in JP-A No. 59-219307 and peroxides as a photopolymerization initiator, as described in JP-A No. 59-219307; the use of monoalkyltriarylborate compounds as a photopolymerization initiator, as described in JP-A Nos. 62-150242, 62-143044, 64-355448; the use of titanocene compounds as a photopolymerization initiator, as described in JP-A Nos. 63-41483, 2-291.
There was further proposed a technique of introducing a tertiary amino group into a monomer (ethylenically unsaturated monomer capable of addition polymerization, in combination with the use of trihalogenomethyl-s-triazine compounds, as described in JP-A No. 1-105238; and a technique of introducing a tertiary amino group into a monomer (ethylenically unsaturated monomer capable of addition polymerization, in combination with the use of trihalogenomethyl-s-triazine compounds and metallocene compounds such as titanocene, as described in JP-A No. 2-127404. In these techniques, however, sufficient plate life was not achieved though improvement in speed was attained.